num = input()
if len(num)==4:
  n1 = num[0]
  n2 = num[1]
  n3 = num[2]
  n4 = num[3]
  S = [int(n1), int(n2), int(n3), int(n4), int(n1+n2), int(n2+n3), int(n3+n4), int(n1+n2+n3), int(n2+n3+n4), int(num)]
  S.sort(reverse = 1)
  for x in S:
    F = True
    for d in range(2, x//2 +1):
      if x%d==0:
        F = False
        break
    if F==True:
        print(x)
        break 
  if F == False:
    print(0)
else:
    print("0")
